Photoconductivity is the phenomena evidenced by the increase in electrical conductivity of a material by the absorption of light or other electromagnetic radiation. Although every insulator and semiconductor material exhibits the phenomena of photoconductivity to some degree, there are relatively few materials that give large enough changes in conductivity with illumination as to be useful commercially.
Photoconductive materials found useful for semiconductor devices are zinc sulfide (ZnS), cadmium sulfide (CdS), cadium selenide (CdSe), cadmium telluride (CdTe), zinc sulfide (ZnS.sub.x Se.sub.1-x), cadmium sulfide selenide (CdS.sub.x Se.sub.1-x), and cadmium sulfide telluride (CdS.sub.x Te.sub.1-x). These materials can be made to provide high photoconductivity gain. To provide high photoconductivity gain the material is typically doped with a metal (usually copper) and a halogen (usually chlorine).
Commercial preparation of photoconductive layers of high photoconductivity gain for semiconductor devices has usually involved first depositing the photoconductive material by evaporation in a high vacuum from one or more evaporant sources. The photoconductive is subsequently doped by baking the layer for about one-half hour or more embedded in chlorine- and copper-dopant powder of the same type. Alternatively, the photoconductive material could be doped by spraying on dopant aqueous solutions of, for example, cadmium chloride and selenothiourea in the presence of air with a heated substrate.
The difficulty is that such doping methods are corrosive and are therefore unsuitable for depositing photoconductive layers integrally with integrated circuits. It is very desirable to use photconductive layers, e.g. photosensitive resistors, in integrated circuits to stop and to start the operation of the circuit by a low intensity light beam. In such instances, it has generally been required that the photoconductive layer be separately formed and then applied to the integrated circuit substrate.
One proposal to overcome the difficulty has been to vapor deposit the dopants or compounds thereof sequentially after the photoconductive layer has been deposited, and thereafter to bake the assembly. For example, to make a cadmium sulfide selenide (CdS.sub.x Se.sub.1-x) layer of high photoconductivity gain, a mixture of cadmium sulfide (CdS) and cadmium (CdSe) is first vapor deposited on a suitable substrate, and then first copper and subsequently cadmium chloride (CdCl.sub.2) are sequentially vapor deposited over the cadmium sulfide selenide layer. The dopants are thereafter incorporated into the cadmium sulfide selenide layer by baking the assembled structure for about an hour or more.
This solution to the problem is, however, time consuming and expensive. Low evaporation and deposition rates, i.e. less than about 10 nanometers per minute, are required. Further, the assembly requires baking for extended periods of time to incorporate the dopants into the photoconductive layer.
The present invention overcomes these problems and difficulties. It provides a simplified process for making high gain photoconductive layers where rapid evaporation and deposition rates can be used and only a very brief heating in air is necessary. Further, the invention provides a method for making photoconductive layers and pads directly on integrated circuit substrates, a method that is compatible with thin-film manufacturing techniques.